i3-migrate-config-to-v4 is a Perl script which migrates your old (< version 4) configuration files to a version 4 config file. The most significant changes are the new commands (see the release notes).
This script will automatically be run by i3 when it detects an old config file. Please migrate your config file as soon as possible. We plan to include this script in all i3 release until 2012-08-01. Afterwards, old config files will no longer be supported.